Tungsten is used to form conductive lines in integrated circuit (IC) devices. Because the resistivity of tungsten is higher than the resistivity of aluminum or copper, the use of tungsten has generally been relegated to filling vertical features such as vias and trenches and local interconnects. Tungsten has an advantage over aluminum and copper in that tungsten does not tend to migrate into and contaminate the silicon or dielectric layers of the device. Copper in particular is highly contaminating and barrier layers must be provided to prevent the copper from contaminating the silicon and dielectric layers. Moreover, aluminum and copper must be sputtered or electroplated onto the substrate, whereas tungsten can be deposited by chemical vapor deposition (CVD).
One problem with tungsten, however, is that it is relatively difficult to get tungsten to start depositing on a substrate (e.g., a semiconductor wafer or oxide layer). Before tungsten will begin to deposit in bulk quantity, a “nucleation” or “seed” layer must be formed on the underlying material. In a CVD process, even after the tungsten-containing gas is introduced into the reaction chamber, a period of time typically elapses before a tungsten nucleation layer begins to form. This time lapse is often referred to as the “nucleation delay”. The nucleation delay may vary from wafer to wafer and from location to location on a single wafer. The resulting tungsten layer is correspondingly non-uniform (i.e., thicker in the areas where nucleation began first and thinner in areas where nucleation began later).
Several techniques have been proposed for reducing the nucleation delay. One standard technique is to bombard the substrate with ions generated by a plasma. The plasma can be formed in a plasma-enhanced chemical vapor deposition (PECVD) chamber. A disadvantage of this technique is that the ion bombardment is highly directional in nature and tends to be directed primarily at flat horizontal surfaces rather than vertical surfaces such as the sidewalls of a trench or via. The nucleation layer thus begins preferentially on the flat horizontal surfaces, with the result that the nucleation is uneven and the step coverage is poor. The resulting tungsten layer tends to look like tungsten layer 10, shown in FIG. 1, which has formed on the horizontal surfaces and on the upper sidewalls of via 12 but is virtually nonexistent on the lower sidewalls and floor of via 12. As the deposition of the tungsten layer continues, tungsten layer 10 may close off the mouth of the via and leave a void in the lower part of the via.
FIG. 2 is a graph showing how the tungsten nucleation layer develops as a function of the number of cycles (which is equivalent to time). The vertical axis is the thickness of the tungsten nucleation layer in Å. The dashed line depicts the formation of the nucleation layer on the plasma-treated horizontal surfaces and the solid line depicts the formation of the nucleation layer on the side walls of via 12. As is evident, the nucleation delay is very different in these two areas.
Accordingly, there is a clear need for improved methods of forming a tungsten nucleation layer and in particular reducing the nucleation delay.